1. Field of the Invention
This invention relates to a garnet film for an ion-implanted magnetic bubble device. More particularly, the present invention relates to a garnet film for an ion-implanted magnetic bubble device which film is specifically suitable for a magnetic bubble device of the type in which at least part of the propagation track of the magnetic bubbles, or at least part of its functional portions such as the transfer gate, generator, etc., is formed by ion implantation. (A device of this kind will be hereinafter referred to as an "ion-implanted device" or an "ion-implanted magnetic bubble device".)
2. Description of the Prior Art
The so-called "permalloy device" whose propagation circuit (propagation track) for the propagation of magnetic bubbles is formed by permalloy patterns has been put into general practical use as a magnetic bubble device, as is known in the art.
If the diameter of the magnetic bubbles is reduced in order to increase the memory density, the sizes and gaps in the transfer pattern must be made extremely small, but such an extremely fine transfer pattern is extremely difficult to fabricate accurately. Moreover, the rotating magnetic field necessary for the transfer must be increased and this is extremely disadvantageous for the operation of the device.
Ion-implanted devices have been proposed to eliminate these problems (e.g., U.S. Pat. No. 3,828,329) in which the propagation circuit is formed by ion-implantation, not by a permalloy film.
Ions such as He.sup.+, Ne.sup.+, H.sup.+, or D.sup.+, etc, are implanted into the upper layer of the desired region within a magnetic garnet film supporting the magnetic bubbles so that a distortion layer having a large lattice constant is formed in the upper layer of the magnetic garnet film, and a layer whose direction of magnetism is parallel to the film surface is formed by the reverse magneto-striction effect.
Accordingly, in this ion-implanted device, the magnetic garnet film has a layer supporting the magnetic bubbles (generally, the lower layer) and an ion-implanted layer driving the magnetic bubbles (generally, the upper layer) and these two layers are used to support and drive the magnetic bubble, respectively.
In conventional permalloy devices, the magnetic garnet film is only used to support the magnetic bubbles and hence it has been necessary to provide a propagation circuit consisting of a permalloy film over the garnet film in order to drive the magnetic bubbles. The ion-implanted device eliminates the necessity of providing a propagation circuit over the garnet film.
The upper limit of the temperature range in which the magnetic bubbles can be smoothly supported and driven without any problems is determined by the lower of the Curie temperatures Tc of the magnetic bubble driving layer and the magnetic bubble supporting layer inside the magnetic garnet film in the ion-implanted device.
In the permalloy device, the Curie temperature Tc of the permalloy film is much higher than that of the magnetic garnet film supporting the magnetic bubbles so that the upper limit of the operating temperature is determined by Tc of the magnetic garnet film.
In the ion-implanted device, on the other hand, it has been found that the Curie temperature Tc of the ion-implanted region of the magnetic garnet film decreases in proportion to the dosage of implanted ions. For example, FIG. 1 illustrates the relation between the ion dosage and the Curie temperature Tc when Ne.sup.+ or He.sup.+ ions are implanted in a magnetic garnet film. In both cases, Tc drops dramatically with the increase in the ion dosage.
For this reason, the upper limit of the operating temperature range of the ion-implanted device is determined by the Curie temperature Tc of the magnetic bubble driving layer formed by implanting ions into the upper layer of a magnetic garnet film.
The Curie temperature Tc of (YSmLuCa).sub.3 (FeGe).sub.5 O.sub.12 that is conventionally used as a typical magnetic garnet film for a magnetic bubble device is about 200.degree. C., but when ion implantation is done under standard conditions (such as the He.sup.+ ion implantation of 1.6.times.10.sup.15 doses), Tc drops to about 170.degree. C. Accordingly, the operating temperature range of the device drops by about 30.degree. C. when compared to a conventional permalloy device and this is a critical problem that must be solved before ion-implanted devices can be put to practical use.